`BinaryHeap` owned iterators should yield items in heap order

[jonhoo]

Currently, both BinaryHeap::into_iter and BinaryHeap::drain yield the heap elements in an arbitrary order. This seems counter-intuitive given the nature of a heap. Sadly, we can't simply change their behavior behind the scenes, as we would have to remove the implementation of DoubleEndedIterator. Should we perhaps add into_ordered_iter and drain_ordered? The implementation is pretty straightforward so I'd be happy to submit a PR.

[clarfon]

Isn't into_sorted_vec().into_iter() sufficient?

[scottmcm]

@clarfon No, because getting the largest k items from that is O(n lg n + k), whereas with into_ordered_iter() it would be O(k lg n).

[ckaran]

@clarfon In addition to what @scottmcm said, into_sorted_vec().into_iter() takes up space proportional to the number of items in the heap because you have to create the Vec first. That means that you've roughly doubled the amount of memory you need, which could be a concern either when you are on more limited systems, or when you're dealing with very large amounts of data.

[clarfon]

@ckaran into_sorted_vec actually doesn't take up any space at all; it reuses the same buffer.